The present invention relates generally to electronic ballast circuits for powering discharge lamps connected in parallel. More particularly, the present invention relates to programmed start electronic ballasts capable of performing dimming operations on multiple discharge lamps connected in parallel.
Electronic ballasts with dimming features are rapidly increasing in popularity, due in part to their capabilities in light output control and energy saving. However, most dimming ballasts can not independently operate a plurality of discharge lamps connected in true parallel mode. In many, if not most, existing configurations lamps are connected in series, which means that if any one lamp fails for some reason or is removed from the circuit all of the accompanying lamps are going to be shut down as well. This results in great expense where it is necessary to replace each inoperable lamp with each such failure even among a large number of lamps in a circuit.
Programmed start ballasts are known in the art for applying a relatively small current to preheat lamp filaments, or cathodes, during a startup process. Particularly where lights are expected to be turned on and off at a high frequency, programmed start ballasts extend the lives of the associated lamps by minimizing glow discharge current. It is not desirable to continue applying the preheat current across the lamp filaments after the lamps have been ignited and are operating at full power, as there is no additional illumination provided and therefore the energy spent is merely wasted. However, in certain situations it is still desirable to have some supplemental current supplied across the lamp filaments to maintain a proper temperature after startup, particularly where a low dimming voltage is provided across the lamps.
Some electronic ballast circuits have been introduced and are known in the prior art to address various combinations of continuous dimming, programmed start with preheat current cutoff, and true parallel lamp operation. However, the additional circuitry required for many of these circuits, particularly with regards to the programmed start ballasts, can be prohibitive with regards to size, complexity and cost.